The present invention relates to a master cylinder device for use, for example, in a braking system or a clutching system for an automotive vehicle, and more particularly relates to a portless type such master cylinder device which is improved with regard to its operational and feeling characteristics.
The present invention has been described in Japanese Patent Application Ser. No. 60-232790 (1985) and 61-021734 (1986), filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Application and the claims and the drawings thereof; a copy is appended to the present application.
Further, the present inventors wish hereby to attract the attention of the examining authorities to copending patent application Ser. No. 918,971, which may be considered to be material to the examination of the present patent application.
In, for example, Japanese Patent Laying Open Publication Ser. No. 56-135348 (1981), which is it not intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law, there is disclosed a portless type master cylinder device, for example for a braking or clutching system for a vehicle such as an automobile, which has a body within which is formed a cylinder bore, with a piston member being fitted in the cylinder bore so as to be slidably movable from an initial axial position therein which said piston member is at when not impelled by the brake or clutch pedal of the vehicle to which this master cylinder device is fitted, a pressure chamber being defined by said piston member in cooperation with the cylinder bore on the one side of said piston member and a reservoir chamber being defined on the other side of said piston member, with a one direction type seal construction being provided for allowing the flow of fluid from said reservoir chamber to said pressure chamber but for preventing the flow of fluid from said pressure chamber to said reservoir chamber, and in which an intake valve communicates between the cylinder chamber and the reservoir chamber, this intake valve being opened when the piston member is in its initial axial position while said intake valve is closed when the piston member moves through more than a determinate relatively small axial distance from its said initial axial position. Thereby, when the brake or clutch pedal of the vehicle is not substantially depressed and accordingly the piston member is in its initial axial position, the intake valve allows brake or clutch fluid to be supplied freely from the reservoir chamber into the cylinder chamber, thus to replenish said cylinder chamber, and also back again, thus to allow excess pressure in said cylinder chamber to be vented to said reservoir chamber. However, when the brake or clutch pedal of the vehicle is progressively depressed, the piston member is progressively displaced from its said initial axial position so as to reduce the volume of said cylinder chamber, and initially said intake valve only slowly and progressively approaches the closed condition, to become completely closed only when said piston member has moved through said determinate relatively small but yet somewhat considerable axial distance from its said initial axial position. Thereafter, the brake or clutch fluid in said pressure chamber is squeezed, to be ejected therefrom towards the braking or clutching system of the vehicle in a per se conventional manner. Such a portless type master cylinder device dispenses with any port or ports in the side of the cylinder bore which are required to be traversed by the land or lands of the piston member during its stroke, substituting the action of the above described valve for the action of said port or ports, and accordingly the durability of the master cylinder device is enhanced.
However, this type of portless type master cylinder device is fraught with problems which will now be outlined. As described above, as the brake or clutch pedal of the vehicle is progressively depressed and the piston member is progressively displaced from its said initial axial position so as to reduce the volume of said cylinder chamber, since initially said intake valve only slowly and progressively approaches the closed condition until said piston member has moved through said determinate relatively small but yet somewhat considerable axial distance from its said initial axial position, thereby during this initial movement of the piston member no actuation of the braking or clutching system for the vehicle is performed, and instead brake or clutch fluid is expelled from the pressure chamber back into the reservoir chamber, which is quite inappropriate at this time. This occurs both when the brake or clutch pedal of the vehicle is depressed at relatively high speed, i.e. during so called dynamic operation, and when said brake or clutch pedal of the vehicle is depressed at relatively low speed, i.e. during so called static operation, and in both cases causes the phenomenon of so called empty or inactive piston stroke, and increase in the pressure in the pressure chamber is delayed to a unduly great extent from the position and the time that the brake or clutch pedal of the vehicle is initially depressed, and furthermore comes on relatively suddenly when it does arrive. Thus, the operational feeling for the brake or clutch pedal is worsened in the case of such a portless type master cylinder device, as compared with a conventional type of master cylinder device, and a good brake or clutch response cannot be obtained.
This phenomenon is illustrated by the graphs shown in FIG. 7. In this figure there are shown a pair of graphs of piston stroke against brake fluid pressure, illustrating the characteristics of such a portless type master cylinder device, and of a conventional type of master cylinder device which is not a portless one: in each case, one said line shows the characteristics in the case of relatively quick brake pedal depression, while the other said line shows the characteristics in the case of relatively slow brake pedal depression. In other words, the one line shows the characteristics under dynamic conditions when the movement of the piston member is carried out relatively rapidly, while the other line shows the characteristics under static conditions when the movement of the piston member is carried out relatively slowly at a time scale large compared with any other time scales of the system. As will be clear from these graphs, for a portless type master cylinder device as described above, in the case of relatively rapid brake pedal operation, the initial portion of the movement of the brake or clutch pedal presents a sudden surge in fluid pressure and therefore in pedal resistance, and accordingly has a deteriorated operational feeling as compared with a conventional type of master cylinder device.